Hinton (1984) described an unusual mutator in D. ananassae which he speculated was a transposable element with previously unknown properties; namely the element could only be phenotypically detected by its effect on eye morphology at 15 non-pleiotropic, non-dosage compensated, non-random loci. This element was known to originate from an X chromosome in a particular stock, ca;px. With the recovery of a spontaneous singed mutation, sn9g, in an ocullar morphology (Om) mutant line derivative, Om(1D)9g, it was possible to investigate his speculation. D. melanogaster singed, sn, DNA probes were used to isolate and recover a D. ananassae singed gene. A comparison of sn9g and wild type singed restriction map implicated a 6 1/2 kb insert as the element responsible for the mutator effect. This insert was shown by Southern blot and in situ hybridization to be repetitive and dispersed. A total of 186 recombinant lines from four X-linked Om loci were examined. 80 were Om and 106 were non-Om; in all instances an in situ hybridization signal, when probed with sn9g insert, was found at appropriate locations on the polytene chromosomes. Linkage was complete and showed that the sn9g insert was homologous to sequences localized at the sites of Om mutants. The most likely hypothesis is that the Om element is inserting at a target site encoded in a control sequence found at eye morphology genes. We intend to study further this phenomenon and to check this hypothesis by characterizing the Om element and insertion target sites. Of particular interest is the possibility of elucidating further the nature of eukaryotic gene structure and control, as well as to increase our knowledge of the nature of mobile genetic elements and their role in spontaneous mutation.